A007814: ;; I.e. compute the number of trailing 0-bits in R20 to R21

    CLR     R21    ;;;;;; 1) Set R21 to zero

LOOP:              ;;;;;;  2) Defines the starting point for a inner loop.
    INC     R21    ;;;;;;  Increases R21 by the unit.

    LSR     R20    ;;;;;;  3) Push to right the bits in R20, the outgoing
                   ;;;;;;;;;; bit is assigned to the carry 1-bit register,
                   ;;;;;;;;;; and Zero takes the place most at left.
                   
    BRCC    LOOP   ;;;;;; 4) If the carry was zero return to Step 2.

    DEC     R21    ;;;;;; 5) When carry be 1 the loop ends, but since each
                   ;;;;;;;;;;;; iteration we added +1 to R21, the last addition
                   ;;;;;;;;;;;; should occur when the BRCC breaks the loop
                   ;;;;;;;;;;;; then the count would be exceeded by 1.
                   ;;;;;;;;;;;; So we restore it.
                      
    RET            ;;;;;;;;;;;; 6) Return the Control and/or the result.
;;